Charts

Understanding chartwork is the basis of good seamanship
Most of the work of charting the world’s oceans and coasts was done in the days of sail, and involved great feats of seamanship and endurance. Charts provided predatory nations with the invaluable knowledge they needed to trade and conquer, so the chartmakers were equipped at great expense with what was then regarded as the latest technology. They had no GPS, of course, and only lead lines in place of electronic depth-sounders; no radio to check the chronometers for longitude; no enclosed power launches to do the inshore work—only sailing ships and open longboats rowed by 4 to 10 men. By today’s standards, they performed miracles with the rudimentary equipment available to them.The main difficulty with drawing a chart is that the paper is flat but the Earth is round. Claudius Ptolemy, the Egyptian astronomer, produced a conic projection of the world in about 230 B.C. However, the projection most useful to sailors up to this very day was the work of the Dutch mathematician, Gerard Mercator, and it didn’t come until the sixteenth century.Mercator’s projection treats the globe as a cylinder, which means that the outlines of land masses become increasingly large and distorted as the poles are approached. Its major contribution to navigation is that it results in a chart on which a straight line drawn between two points represents the constant compass course to be steered all the way. Although this results in a slightly longer course over the curved Earth’s surface, it has the overwhelming advantage of simplicity.

Mercator’s charts result from the round surface of the Earth being projected onto a cylinder having the same diameter as the projected globe but touching it only at the equator.

Furthermore, a Mercator chart provides an easy way to measure distances. The parallels of the grid are spaced so that 1 minute of latitude always equals 1 nautical mile, and the parallels are conveniently measured on either side of the chart.The chart projection that provides the shortest route between two points is the great-circle, or gnomonic, projection, where one point on the Earth’s surface is used as a center from which land and water are increasingly distorted outward. A straight line drawn on a great-circle chart crosses each meridian at a slightly different angle, theoretically resulting in slight but continuous course changes for any vessel following it. In practice, vessels change course more substantially at every 10-degree meridian, with little loss.Only seldom can a sailboat follow a great-circle course exactly, but she should follow it as closely as circumstances allow, especially in higher latitudes because the savings in distance are greater the nearer she approaches the poles. The most favorable tack for a boat beating to windward is the one that takes her closest to the great-circle course.The amount of information contained in a nautical chart—from bottom depths and types to navigational aids and dangers—is extraordinary. There is so much detail, often presented in a special shorthand form, that it takes study and practice to extract the most benefit from a chart. Efficient chartwork, in fact, is the basis of good seamanship.See also Chart Datums; Chart Scales; Chart Stowage; Depth-Sounders; Electronic Charts; Rhumb Lines.

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